Although several organisms are capable of rolling locomotion, no organisms are known to use true wheels or propellers to locomote, with the exception of the corkscrew-like flagella of many prokaryotes. Biologists have offered several explanations for the apparent absence of biological wheels, and wheeled creatures have appeared often in speculative fiction.
Given the ubiquity of wheels in human technology, and the existence of biological analogues of many other technologies (such as wings and lenses), the lack of wheels in nature has seemed, to many scientists, to demand explanation—and the phenomenon is broadly explained by two factors: first, there are several developmental and evolutionary obstacles to the advent of a wheel by natural selection, and secondly, wheels have several drawbacks relative to other means of propulsion (such as walking, running, or slithering) in natural environments, which would tend to preclude their evolution. This environment-specific disadvantage has also led humans in certain regions to abandon wheels at least once in history.
Known instances of rotation in biology There exist two distinct modes of locomotion using rotation: first, simple rolling; and second, the use of wheels or propellers, which spin on an axle or shaft, relative to a fixed body. While many creatures employ the former mode, the latter is restricted to microscopic, single-celled organisms.
Rolling
Some organisms use rolling as a means of locomotion. These examples do not constitute the use of a wheel, as the organism rotates as a whole, rather than employing separate parts which rotate independently.
Several species of elongate organisms form their bodies into a loop to roll, including certain caterpillars (which do so to escape danger), tiger beetle larvae, myriapods, mantis shrimp, Armadillidiidae, and Mount Lyell salamanders. Other species adopt more spherical postures, primarily to protect their bodies from predators; this posture has been seen in pangolins, wheel spiders, hedgehogs, armadillos, armadillo girdled lizards, isopods, and fossilized trilobites. Pangolins and wheel spiders have been observed to purposely roll away from predators. These species may roll passively (under the influence of gravity or wind) or actively, typically by altering their shape to generate a propulsive force.
Tumbleweeds, which are the above-ground portions of certain plants, separate from their root structure and roll in the wind to distribute their seeds. These plants are found especially in open plain environments. The most well-known of these include Kali tragus (also known as Salsola tragus), or prickly Russian thistle, which arrived in North America in the late 19th century, and gained a reputation as a noxious weed. Fungi of the genus Bovista are known to use the same strategy to disperse their spores.
Rotifers are a phylum of microscopic but multi-celled animals, typically found in freshwater environments. Although the Latin name rotifer means 'wheel-bearer', these organisms do not have any rotating structures, but rather a ring of rhythmically beating cilia used for feeding and propulsion.
Keratinocytes, a type of skin cell, migrate with a rolling motion during the process of wound healing. These cells serve to form a barrier against pathogens and moisture loss through wounded tissue.
Dung beetles form spherical balls of animal excrement, which they roll with their bodies, generally by walking backwards and pushing the ball with their rear legs. Phylogenetic analysis indicates that this rolling behavior evolved independently several times. The behavior of these beetles was noted in ancient Egyptian culture, which imparted sacred significance to their activities. Although it is the dung ball that rolls rather than the beetle itself, the beetles face many of the same mechanical difficulties that rolling organisms contend with.
Free rotation Macroscopic
Among animals, there exists a single known example of an apparently freely rotating structure, though it is used for digestion rather than propulsion: the crystalline style of certain bivalves and gastropods. The style consists of a transparent glycoprotein rod which is continuously formed in a cilia-lined sac and extends into the stomach. The cilia rotate the rod, so that it becomes wrapped in strands of mucus. As the rod slowly dissolves in the stomach, it releases digestive enzymes. Estimates of the speed of rotation of the style in vivo vary significantly, and it is unclear if the style is rotated continuously or intermittently.
Microscopic There are two known examples of molecular-scale rotating structures used by living cells. The first, ATP synthase, is a transmembrane enzyme used in the process of energy storage and transfer in all known organisms. The electron transport chain (ETC) establishes a proton gradient across the cell membrane in prokaryotes and the mitochondrial membrane in eukaryotes, such that there is a buildup of protons outside the cell (or in the intermembrane space of mitochondria). This allows for the energetically-favorable transport of protons across the membrane into the cytoplasm (or mitochondrial matrix) through special subunits of ATP synthase, whose subsequent rotation powers the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP)—the energy storage molecule—via the addition of one inorganic phosphate (Pi). The mechanism of rotation bears some similarity to the flagellar motors discussed below. ATP synthase is thought to have arisen by modular evolution, in which two subunits with their own functions have become associated and gained a new functionality.
The only known example of a biological "wheel" or "propeller"—a system capable of providing continuous propulsive torque about a fixed body—is the flagellum, a corkscrew-like tail used by single-celled prokaryotes for propulsion. The bacterial flagellum is the best known example. About half of all known bacteria have at least one flagellum; thus, given the ubiquity of bacteria, rotation may in fact be the most common form of locomotion used by living systems—though its use is restricted to the microscopic environment.
At the base of the bacterial flagellum, where it enters the cell membrane, a motor protein acts as a rotary engine. The engine is powered by proton motive force, i.e. by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism. (In species of the genus Vibrio, there are two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump.) Flagella are quite efficient, allowing bacteria to move at speeds of up to 60 cell lengths per second. The rotary motor at the base of the flagellum is similar in structure to ATP synthase. Spirillum bacteria have helical bodies with flagella at either end, and they spin about the central axis of their bodies as they move through the water.
Archaea, a group of prokaryotes separate from bacteria, also feature flagella—known as archaella—driven by rotary motor proteins, which are structurally and evolutionarily distinct from bacterial flagella: whereas bacterial flagella evolved from the bacterial Type III secretion system, archaella appear to have evolved from type IV pili.
Some eukaryotic cells, such as the protist Euglena and animal sperm, possess a convergent, evolutionarily distinct flagellum-like structure known as a cilium or undulipodium. Unlike bacterial flagella however, these structures do not rotate at the base; rather, they bend in such a way that the tip whips in a circle.
Navicula, a type of diatom, may locomote using a band of free-flowing mucilage strands, in the manner of a tracked vehicle. Navicula Diatom: Youtube videoJ Microbiol Methods. 2013 Mar;92(3):349–354. doi: 10.1016/j.mimet.2013.01.006. Epub 2013 Jan 18. Semi-circular microgrooves to observe active movements of individual Navicula pavillardii cells. Umemura K1, Haneda T, Tanabe M, Suzuki A, Kumashiro Y, Itoga K, Okano T, Mayama S.
Biological barriers to wheeled organisms
The absence of wheels in nature is frequently attributed to constraints imposed by biology: natural selection constrains the evolutionary paths available to species, and the processes by which multicellular organisms grow and develop may not permit the construction of a functioning wheel.
Evolutionary constraints
The processes of evolution can help explain why wheeled locomotion has not evolved in multicellular organisms, as a complex structure or system will not evolve if its incomplete form does not serve to propagate the organism's genes.
Adaptations are produced incrementally through natural selection, so major phenotypic changes will not usually spread within populations if they decrease the fitness of individuals. Although changes that do not affect fitness can spread through genetic drift, and detrimental changes can spread under some circumstances, large changes that require multiple steps will occur only if the intermediate stages increase fitness. Richard Dawkins describes the matter: "The wheel may be one of those cases where the engineering solution can be seen in plain view, yet be unattainable in evolution because it lies [on] the other side of a deep valley, cutting unbridgeably across the massif of Mount Improbable." In such a fitness landscape, wheels might sit on a highly favorable "peak", but the valley around that peak may be too deep or wide for the gene pool to migrate across by genetic drift or natural selection. Stephen Jay Gould notes that biological adaptation is limited to working with available components, commenting that "wheels work well, but animals are debarred from building them by structural constraints inherited as an evolutionary legacy".
Natural selection therefore explains why wheels are an unlikely solution to the problem of locomotion: a partially evolved wheel, missing one or more key features, would probably not impart an advantage to an organism. The exception to this is the flagellum, the only known example of a freely rotating propulsive system in biology; in the evolution of flagella, individual components were recruited from older structures, where they performed tasks unrelated to propulsion. The basal body that is now the rotary motor, for instance, might have evolved from a structure used by the bacterium to inject toxins into other cells. This recruitment of previously evolved structures to serve new functions is called exaptation.
Molecular biologist Robin Holliday has written that the absence of biological wheels argues against creationist or intelligent design accounts of the diversity of life, because an intelligent creator—free of the limitations imposed by evolution—would be expected to deploy wheels wherever they would be of use.
Developmental and anatomical constraints Using human manufacturing processes, wheeled systems of varying complexity have proven fairly simple to construct, and issues of power transmission and friction have proven tractable. It is not clear, however, that the vastly different processes of embryonic development are suited to—or even capable of—producing a functioning wheel, for reasons described below.
The greatest anatomical impediment to wheeled multicellular organisms is the interface between the static and rotating components of the wheel. In either a passive or driven case, the wheel (and possibly axle) must be able to rotate freely relative to the rest of the machine or organism. Unlike animal joints, which have a limited range of motion, a wheel must be able to rotate through an arbitrary angle without ever needing to be "unwound". As such, wheels cannot be permanently attached to the rest of the machine or organism that uses them. There are several functional problems created by this requirement, though these may be partly surmountable.
Power transmission to driven wheels